46 PROCEEDINGS OF THE NATIONAL MUSEUM. vou. xxxvv. 
a gently curved cardinal margin. The growth from umbo directly 
toward the front is commonly nearly twice as rapid as the growth 
parallel to the hinge. The resulting effect is the production of a shell 
whose width is less than twice its length. 
The important dimensions of the contour are those from beak to 
front along the median line, which is the length, and the greatest 
diameter attained in a direction at right angles to the length, which 
is the width. 
Taking these two diameters, we may define the forms of contour to 
be as follows: : 
Form alpha—twtLength and width, nearly equal, greatest width 
about half way from beak to front. 
Form beta—Width greater than length, shell elliptical, greatest 
width subcentral. 
Form gamma.—tLength and width subequal, but greatest width 
near front margin. 
Form delta.—Leneth and width subequal, but greatest width near 
hinge line. 
For all the above four forms the two valves are shghtly convex. 
As the growth accelerates in the direction from beak to front, the 
surface of the brachial valve arches upward making that valve gib- 
bous while the opposite valve remains nearly flat and becomes in some 
cases concave at the margins. The actual contour around the plane 
of meeting of the edges of the two valves does not greatly depart 
from the form already designated, although the decrease in relative 
rapidity of growth in a lateral direction results in placing the greatest 
width up above the middle near the hinge line. This mode of growth 
is seen in species D). carinata and DP. virginia, resulting in two con- 
tour forms. 
Porm epsiton.—Actual contour oval with width greater than length, 
but the brachial valve strongly arched, the pedicle valve flat or 
shghtly convex, giving to the brachial valve an outlne approaching 
the form delta and to the pedicle valve an elliptical form like deta, 
but with the greatest width near hinge line. This form is seen in 
D. carimata var. epsilon. 
Form zeta.—Same as epsilon except that the pedicle valve is con- 
cave, particularly along the median line from beak to front and with 
the margin of shell scooping outward as seen in typical ). carinata. 
These contour forms are not evolutional but affect specimens at 
each stage of their history except in the following particulars, viz: 
The form alpha dominates throughout the Chemung series; form 
beta is rare among the earliest small forms, but as the size increases 
the form beta dominates in the Cayuta member for the zone immedi- 
ately following the Dalmanella danbyi stage. When the second 
stage of increase of size is attained the form gamma becomes domi- 
